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    An approach is described for recording and characterizing giant nerve fibre activity in intact, freely moving earthworms. 2. Medial giant fibre (MGF) spikes were conducted in an anterior-posterior direction at a mean rate of 32.2 m/s; lateral giant fibre (LGF) spikes were conducted in a posterior-anterior direction at a mean rate of 12.5 m/s. 3. Rates of giant fibre spike conduction and maximal frequencies of firing (up to 500/s) in intact animals were higher than values previously reported in isolated preparations. 4. MGF spikes were followed 1:1 by presumed giant motor axon spikes and facilitating muscle potentials. 5. Single MGF or LGF spikes evoked by applying tactile stimulation were not accompanied by longitudinal contraction, but a series of two or more MGF spikes or three or more LGF spikes were accompanied by such contractions. 6. MGF and LGF spikes occurred infrequently during locomotory movements in the absence of any experimenter-applied stimulation, suggesting that sensory inputs associated with normal locomotion over an irregular substrate are sufficient to excite giant fibres.

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